The female suspect in the mass shooting earlier this week that left 14 people dead in San Bernadino apparently pledged allegiance to a leader of Islamic State militant group, two U.S. government sources said Friday.

Tashfeen Malik, 27, and her husband, Syed Rizwan Farook, 28, were killed in a gun battle with police hours after the Wednesday massacre at a holiday party at a social services agency.

U.S. authorities are evaluating evidence that Malik, a Pakistani native who had been living in Saudi Arabia when she married Farook, a U.S. native, had pledged allegiance to Islamic State leader Abu Bakr al-Baghdadi, two U.S. officials told Reuters.

They said the finding, if confirmed, could be a "game changer" in the investigation.

Prayer vigils

Meanwhile, prayer vigils were held at several sites in California Thursday evening following the shooting rampage that also wounded 21.

Officials say they still do not know what prompted Farook and Malik to open fire on the man's co-workers.

Reports say Farook left the party Wednesday after arguing with a co-worker. He and his wife, wearing assault style clothing, later returned to the party armed with semiautomatic rifles and handguns.

The police discovered 12 pipe bombs, bomb-making materials and thousands of rounds of ammunition in the couple's apartment.

The two were killed in a shootout with police in the city of San Bernardino in a residential neighborhood close to Farook's office. His co-workers described him as mild-mannered and well-liked.

San Bernardino is about 100 kilometers (60 miles) east of Los Angeles.

San Bernardino Police Chief Jarrod Burguan said the pair fired 75 bullets and left behind a toy car rigged with three pipe bombs that failed to explode.

Farook from US Midwest

Farook, who was born in Chicago, was a government health inspector. His wife, who was from Pakistan, was in the U.S. on a fiance visa. Neither was known to have a criminal record and they were not on any watch list. The couple had a 6-month-old child.

"I can assure the American people we're going to get to the bottom of this," President Barack Obama said Thursday.

Obama, police and the Federal Bureau of Investigation have not ruled the atack terrorism. There is a possibility the killings were part of a work-related dispute.

But the assistant chief of the FBI's Los Angeles office, David Bowdich, said it was clearly planned.

"There was obviously a mission here. We know that. We do not know why. We don't know if this was the intended target or if there was something that triggered him to do this immediately," Bowdich said. He also said the FBI is scouring the couple's digital devices for evidence.

Jesus Gonzales, center left, who has been separated with his wife since Wednesday's shooting, is comforted by local church members including Jose Gomez, center right, in San Bernardino, Calif., Dec. 3, 2015.

San Bernardino Police Lieutenant Mike Madden was the first officer to arrive on the scene of the massacre. He told reporters that the carnage was "unspeakable" and that those who had not been injured had "pure panic" on their faces.

After the attack, the couple escaped in a black SUV that police spotted in the nearby city of Redlands, setting off a gunfight. Officers fired nearly 400 rounds of ammunition at the car, killing the couple on the spot.

Two officers were wounded, but not seriously.

Muslim community concerned

The massacre stunned the U.S. Muslim community, whose leaders say they are as heartbroken and as horrified as everyone else.

"There's a lot of anxiety among American-Muslims because we have seen it in the past," said Nihad Awad, the head of the Council on American-Islamic Relations. "We've seen what jumping to conclusions means and how it impacted our lives."

Awad said he spent part of Thursday answering telephone calls from parents worried about whether it was safe to send their children to school because of a possible backlash.

A police officer picks up a weapon from the scene of the investigation around the area of the SUV vehicle where two suspects were shot by police following a mass shooting in San Bernardino, California, Dec. 3, 2015.

Wednesday's mass shooting was the worst in the U.S. since a mentally ill man gunned down 26 children and teachers in a Connecticut elementary school in 2012. It also was the 353rd time this year that four people or more were shot dead in a single incident, and is certain to add more fuel to the debate over gun control.

The weapons used in San Bernardino were bought legally.

President Obama said it is just too easy for people to buy guns in the United States. He said what happened Wednesday should spur lawmakers in Washington "to take basic steps to make it harder, not impossible, to get weapons."

Many Republicans in Congress resist tighter gun laws and point out that the right to own firearms is protected by the second amendment to the U.S. Constitution.